This invention relates generally to a method for producing landplaster, and more particularly to a method for increasing the efficiency of a conventional landplaster production facility though more effective collection of gypsum fines.
Natural gypsum is a mineral found in large deposits in open pits and underground. One common method for processing the gypsum rock is to use a series of primary and secondary crushers and screens to obtain a desired size or size range of the product. The particle size of the crushed and screened gypsum rock will vary with the requirements of the landplaster producer, but generally falls within a range of 11/2 to 2" top size to 1/4 to 1/2" bottom size.
Crushed gypsum rock is difficult to handle in that it does not flow freely. Conventional landplaster production systems generally include a dryer that dries the purified gypsum rock prior to fine grinding to make it easier to handle. This drying process generally includes a rotary dryer provided with screw flights to prevent the generation of dust, and to move the material from an input end towards an output end. In a typical facility, the dryer is temperature controlled so that its internal temperature does not exceed 120.degree. F. After being subjected to primary and secondary crushing and screening operations, fine grinding of mined gypsum rock is generally accomplished by air-swept roller mills fitted with integral air separators for better control of particle size. A preferred type of roller mill is sold under the designation Raymond mill. The Raymond mill is designed to accept an optimum input size of -3/4" of feed material. In the past few years, high-energy impact mills plus air classifiers also have been used. One drawback of such conventional rock processing systems is that the air classifiers are limited to removing only the unwanted dust from the material flow.
In a conventional landplaster production system, the rotary dryer is approximately 110 feet long and turns at approximately 4 to 5 revolutions per minute, with a maximum drying capacity of 81.63 tons (90 metric tonnes) per hour, while the Raymond mill has a maximum grinding capacity of 63.49 tons (70 metric tonnes) per hour. Therefore, the production rate for the entire system is limited by the Raymond mill output.
It has been found that conventional handling of gypsum rock dust or fines of -100 mesh, i.e. material that is 100 .mu.m (0.0039 inches) or less in size, acts as a negative influence on the production rate of the landplaster production system. This is because the fines are fed into the Raymond mill along with larger sized particles of the dried gypsum rock, where the fines take up space, clogging the Raymond mill. As the fines do not need to be ground, in effect the Raymond mill consumes power stirring the fines around without decreasing or otherwise affecting their size. The presence of fines in the Raymond mill decreases the available grinding capacity for larger particles which actually require grinding, and thus detracts from the production capacity of the entire system. Excessive amounts of fines in the Raymond mill also tend to choke the air flow out the mill discharge, further impeding production.
Attempts to solve this problem include speeding up the grinding mill, however that effort resulted in significantly higher mill maintenance costs which offset the increase in production. Other attempted solutions to this problem include efforts to reduce the amount of fines introduced into the Raymond mill. While the primary and secondary crushing steps include screening to reduce the amount of fines generated, significant amounts of fines are still found in the feed to the Raymond mill. In another effort to reduce fines in the roller mill feed, the prior art production system also includes a dust housing and a dust collector with a specifically designed duct connection to eliminate fines from the production system. However, this equivalent has also failed to satisfactorily limit the amount of fines in the roller mill feed.
Thus, the prior art system does not solve the production capacity problems discussed above. Therefore, the Raymond mill still consumes excess power stirring the fines without affecting their size. Second, removing the fines using current methods does not increase the prior art system production capacity. Despite the above-listed attempts, the system production capacity is still limited by the output of the Raymond mill.
Thus, there exist a need to improve the production capacity of a landplaster production system. There is further a need to improve the processing of fines in the system to reduce the amount of fines passed to the Raymond mill.
Accordingly, a first object of the present invention is to provide an improved landplaster production system with increased production capacity and efficiency.
A further object is to provide an improved landplaster production system wherein a significant proportion of fines are separated from the main flow of material prior to grinding in the roller mill.
Other objects and advantages of the invention will become more apparent from the following description and accompanying drawings.